The invention relates to remote control systems responsive to transmitted control signals for selectively performing a plurality of independent functions. More particularly, the invention relates to remote control systems of the foregoing type having improved noise immunity means to enable the system to distinguish between intentionally generated control signals and spurious signals. While not to be so restricted, the invention will be discussed in terms of a remote control system for a television receiver.
Ultrasonic remote control systems have for years been a popular control medium for television receivers. A typical system consists of a viewer actuated transmitter for producing ultrasonic control signals at discrete frequencies and a remote receiver in the television set for receiving and decoding the control signals. The transmitter may be either electronic, and include an oscillator, or mechanical and employ rods of selected length and material which, when mechanically struck, produce distinctive ultrasonic frequencies corresponding to the selected function. A transducer in the remote receiver converts the transmitted ultrasonic information into corresponding electrical information which is supplied to a series of resonant circuits, each responsive to a particular control signal frequency. The resonant circuits thus determine which control signal has been received and activate appropriate utilization circuits to perform the selected function in the television receiver.
More recently, remote control receivers have been proposed wherein digital techniques are employed to count the cycles of an ultrasonic signal received during a predetermined timing interval for identifying the transmitted frequency and the corresponding function to be controlled. Exemplary of the foregoing are the systems disclosed in U.S. Pat. Nos. 3,855,575 to Leuschner et al and 3,611,297 to Kramer et al. In other recent remote control receivers each controlled function uniquely corresponds to a continuous band of transmitted frequencies. Decoding circuits, in response to a counter-derived control frequency, determine the frequency band in which the received signal lies and the corresponding function to be controlled. U.S. Pat. Nos. 3,980,956 and 4,023,105 to Woolling, Jr. and 4,006,462 to Podowski et al are exemplary of the latter type of system.
In the ultrasonic spectrum, noise produced, for example, by the jingling of keys or dropping of coins on hard surfaces may cause false actuation of individual functions of the television set by the remote receiver. Recognizing that control signals and noise are distinguishable on the basis of amplitude and duration, a common form of noise immunization uses RC networks for integrating the relatively long duration control signals. Noise or other short duration signals are generally incapable of actuating the utilization devices. Another form of noise immunization uses noise information to reverse bias the remote amplifier, and in effect, preclude operation of the system in the presence of noise. The previously mentioned Kramer patent employs a noise suppressor circuit which requires that a valid control signal have a constant amplitude for a fixed period of time. The noise suppressor prevents the counter from responding to noise related pulses thereby providing noise immunization. In a similar manner, the remote receiver disclosed in the Leuschner et al referred to above includes means whereby, when interfering noise is detected, the system is shut down to prevent counting of the noise related pulses.
As previously discussed, U.S. Pat. Nos. 3,980,956 and 4,006,462 disclose remote control systems wherein each controlled function corresponds to a continuous band of frequencies. In these systems, a digital counter is used to repetitively derive the frequency of a transmitted control signal during each of a series of timing intervals. The determination that a derived frequency falls within one of the frequency bands presumptively indicates that the viewer has selected the corresponding controlled function for actuation. Noise immunity is achieved by requiring that, before a controlled function is actuated, the derived frequency fall within the same frequency band for a predetermined number of timing intervals. While further noise immunity may be achieved by reducing the bandwidth of the controlled function related frequency bands, the increased selectivity of the resulting narrowband system would not be compatible with the frequency tolerances to be expected in an ultrasonic transmitter intended for use in the consumer electronics market. For example, in addition to frequency deviations resulting from manufacturing tolerances, it is not uncommon for the frequency of an ultrasonic transmitter to drift substantially with age. Moreover, slight physical movements of the transmitter by the viewer can result in frequency deviations of the transmitted control signals. A highly selective narrowband system would be incapable of accommodating these factors and would lead to decreased reliability of positive function selection by the viewer. Thus, although a narrow bandwidth system is desirable from a noise immunity viewpoint, a broadband system on the other hand allows for increased system reliability by more effectively accommodating normal frequency deviations. The present invention resolves these apparently conflicting design considerations by providing a remote control receiver advantageously combining the selectivity characteristics of a broadband system with the improved noise immunity of a narrowband system.